The present invention relates to a method and apparatus for cooling solid state devices; and, more particularly, a method and apparatus for providing a good heat dissipation from a thermally isolated device, such as Silicon on Insulator (SOI) devices, by using a Peltier junction.
It is well known that the speed and power output capabilities of a variety of semiconductor devices are dependent upon the carrier mobility and transit time response of these devices. It is further well known that these latter characteristics are temperature dependent. Thus, when these devices are driven beyond their upper power limits or current ratings, they may heat up to in turn reduce the levels of carrier mobility and transit time therein to thereby cause a concurrent reduction in speed and power output of these devices, and in some cases device burn-out. Thus, when these devices are mounted on lead frames or headers and encapsulated in a package, good heat transfer and heat dissipation from these devices have always been primary considerations in package design.
In the past, the use of passive cooling methods having good heat transfer characteristics and thermal dissipation characteristics for lead frames, component headers and the like has frequently been adequate for providing satisfactory heat dissipation and heat transfer for electronic devices operating within certain prescribed and normal limits of power output and operational speed. However, with the recent rapid advances in the art of integrated circuit memories, for example, where several million semiconductor devices are on a chip, there is a definite need to provide an active enhanced cooling capability for these devices in addition to the above types of passive cooling methods.
Thus, active cooling structures may be used to provide an additional kind and degree of cooling for these semiconductor devices. This enhanced cooling may be desirable, for example, to allow these semiconductor devices to operate at even greater speeds and higher powers (and higher speed-power-product figure of merit) than were heretofore possible. These devices not only include integrated circuits as indicated above having many millions of individual power transistors which, with the help of additional active cooling, would be capable of operating at significantly greater speeds and output powers than they are presently capable of operating using only passive cooling methods.
The specific type of cooling employed herein and the technical field of the present invention is that of Peltier cooling capable of using either metal-metal or metal semiconductor Peltier junctions. These junctions produce either a cooling or heating effect at the metal-metal or metal-semiconductor interface, depending upon the direction of current flowing across this interface. More specifically, there is either a cooling or heating effect depending upon the direction or current flowing thereacross. This effect had been described in many prior art patents and publications and is based upon a discovery made by Jean Peltier in 1834.
The Silicon on Insulator (SOI) devices are isolated from the bulk silicon by an insulator, usually, silicon oxide. The insulator, however, not only isolates the devices electrically, but also thermally as well, thereby making them run hotter than standard devices. Normal bulk cooling techniques are not adequate in these instances as the insulator acts as a thermal barrier.
The temperature effects of the SOI devices, e.g., NMOS and PMOS devices are well known. The mobility of the carrier is inversely proportional to the temperature as approximately Txe2x88x921.5. Thus the mobility decreases and, therefore, the resistivity increases by about 40 percent for each 100xc2x0 C. temperature rise. Both the drain current and threshold voltage decrease with temperature. The transition for weak to strong inversion broadens. These effects make lower voltage devices difficult to fabricate. However at cooler temperatures the above effects reverse and the transition from weak to strong inversion becomes abrupt. This makes lower voltage device operation possible.
It is, therefore, an object of the present invention to provide an improved cooling means for a thermally isolated device.
In accordance with one aspect of the present invention, there is an electronic device, comprising:
a substrate;
an active region on top of and isolated from the substrate;
a first material region on top of and/or adjacent to and electrically isolated from but thermally coupled to the active region;
a second material region attached to a surface of the first material region to form an interface defining a Peltier cooling junction therebetween; and
a current source connected in series to the first and the second material regions, whereby the current source produces a cooling effect at the Peltier cooling junction.